The structure of a typical semiconductor package is comprised of a semiconductor chip 1A, a die pad 2A, leads 3A and a package body 4A, as shown in FIGS. 1 and 2. In order to attach chip 1A to die pad 2A, a layer of silver paste 5A can be applied between semiconductor chip 1A and die pad 2A. Different coefficients of thermal expansion exist for the chip 1A, the silver paste 5A, the die pad 2A and packaging resins forming the package body 4A, since the materials used to make them are different.
The steps of die attaching, molding and post mold curing are performed under a high temperature environment of from 150.degree. C. to 175.degree. C. during the packaging of semiconductor units. The differences in the coefficients of thermal expansion of the chip, die pad, silver paste and packaging resin result in the variations of thermal stress by heating and cooling during the packaging process. Accordingly, crack and delamination occur between chip 1A and package body 4A, chip 1A and silver paste 5A, silver paste 5A and the upper surface of die pad 2A, and also between the bottom surface of die pad 2A and package body 4A in the finished semiconductor package, thus deleteriously affecting the reliability of the products.
In addition, recently, high speed and multi-functions developments have been made in computer-based electronic products. Therefore, the capacity of the chips have had to be increased. In order to match the tendency in increasing chip capacity efforts must be made in making larger chips. When the size of the chips is increased, the die pad becomes accordingly larger, and the amount of silver paste coated thereon has to be increased as well. The larger the chips, die pad and silver paste, the greater the thermal stress. With increased thermal stress, the phenomena of cracking and delamination on the finished semiconductor packages also occur easier. Thus, the quality reliability of the products is significantly lowered.
One of the approaches in resolving the above-mentioned problem is to change the design of the die pad, such that the stress effect can be decreased. Various modified die pads are described in detail as follows:
1. Open-Slot Type
As shown in FIG. 3(A), a plurality of slots (or holes) were opened on the die pad to enhance the holding force of the die pad. The stresses are enabled to be absorbed by these slots, so that the disadvantageous effects produced on the die pad by the stress between chips and the die pad and between the die pad and the packaging resin are decreased. Thus, the occurrence of crack and delamination is reduced. However, as shown in FIG. 3(B), since there are a number of slots on such die pad, when chips are attached to the die pad, in order to prevent the silver paste from bleeding through these slots and contaminating the packaging equipment, or even resulting in cracking or secondary delamination, the pasting amount and pasting position of the silver paste must be severely controlled. In order to strictly control the pasting amount and pasting position of the silver paste, the pasting head must be particularly designed. Therefore, not only is the producing cost increased, the bleeding problem of the silver paste still cannot be entirely eliminated. Thus, there are some improvements that have to be made for this type of die pad, to prevent too much pasting from resulting in bleeding. Improvements have to be made to prevent not enough pasting affecting the attachment of the chip to the die pad, or improper pasting leaving spaces between the chip and the die pad resulting in the forming of voids during the molding or injection process, such that cracks or delamination easily occur between the chip and the die pad.
2. Hollow Type
As shown in FIG. 4(A), a rectangular trough is formed in the center of the die pad. Both the stress effects between chips and the die pad and between the die pad and packaging resin are decreased via reducing the area of the die pad and consequently reducing the amount of the silver paste needed. Meanwhile, the formation of the rectangular trough greatly reduces the material needed for the die pad; and provides better results for decreasing stress effect than the above mentioned open-slots type. Although the area of the die pad is greatly reduced, the problem of silver paste bleeding from the edge of the rectangular trough (see FIG. 4(B)) still exists. However, since the pasting amount and pasting position still have to be strictly controlled, complexity and difficulty of the process are increased. In addition, like the above mentioned open-slot die pad, not enough pasting or an improper pasting position will leave spaces between the chip and the die pad, form voids during the molding or injection process, and result in the occurrence of cracks. This can even result in a failure in the attachment between the chip and the die pad since parts of the chip and the die pad are not coated. Therefore, improvements still have to be made.
3. Crossed Type
As shown in FIG. 5(A), the die pad is comprised of two crossed pad plates. The material used for the crossed type die pad is reduced as compared to the hollow type; as well, the influence produced by the stress effect is decreased due to the reduction of the area and the shrinkage of the attachment area between the chip and the die pad. However, as shown in FIG. 5(B), the above mentioned problem of contaminating the packaging equipment resulted from bleeding of the silver paste and the problem of secondary delamination still exist. Meanwhile, when using this type of die pad, after die attaching, most of the circumference of the chip is not fully attached to the die pad. Since cracks may easily occur during the process of die attachment, this is not an ideal type of die pad.
4. Shrunken Type
As shown in FIG. 6(A), the die pad is basically comprised of a slug-like pad plate, with an enlarged portion formed in the approximate center position of said slug-like pad plate. Since the area of the shrunken type die pad is smaller than the crossed type, the cost of the material is less. As well, the smaller area receives a lesser impact from the remaining stress when compared to that of the crossed type. However, the problem of silver paste bleeding still exists and the amount of pasting still has to be strictly controlled (as shown in FIG. 6(B)) as in the above-mentioned other three die pads. Furthermore, since the area is smaller, the horizontal position of the attachment of the chip to the die pad must be carefully controlled during the die attachment process, such that the proper support and horizontal orientation of the chips can be obtained in order to meet the requirements of the wire bonding process.
In addition to the above mentioned problems, there is a common disadvantage in those disclosed four different types of die pad. That is, since the dimension of the conventional die pad along the longitudinal extension direction (Y direction as shown in FIG. 3(A)) of the tie bar attached thereon is longer, greater stress effect is produced when the temperature varies, easily resulting in the deformation of the die pad in the Y direction. The occurrence of this formation will cause cracks and delamination. This is a major defect that needs to solve.